Known processes for manufacturing articles in composite materials use the technology of heat compression molding, in which measured quantities of materials comprising a heat-hardening matrix reinforced with fibers of rigid materials are inserted into molds made of steel or aluminum suitable to a confer a predetermined shape to the article. This technology, created for molding articles reinforced with glass fibers but today increasingly used also for articles reinforced with carbon fibers, allows to obtain components characterized by low weight and excellent mechanical properties, as well as good dimensional tolerances.
The heat compression molding technology is very suitable for the manufacture of articles substantially free from undercut details, obtainable with molds made up of two overlapping parts inside which the shape of the article is formed in negative and in two portions.
In a typical process of heat compression molding for articles of composite material based on carbon fibers, semi-finished products made of heat-hardening resins and carbon fibers are used. These semi-finished products, manufactured in processes separate from the compression molding process, are in the form of pre-impregnated sheets known in the industry with the English acronym “SMC” (Sheet Molding Compound).
The SMC semi-finished products generally comprise a plurality of pre-impregnated sheets superimposed so as to form a single block. Before being loaded into the mold, an SMC semi-finished products can be advantageously subjected to a heating treatment, for example by means of infrared rays, which facilitates its arrangement in the mold and allows to reduce the curing time of the heat-hardening resin.
In order to control features such as the dimensional shrinkage during the hot crosslinking phase, the surface finishing, the ease of detachment from the molds and the behavior in fire, the heat-hardening resins are usually charged with mineral charges, lubricants, release agents, flame retardants and the like. Calcium carbonate, kaolin, stearates, oxides and hydrates of calcium, magnesium, aluminum are some of the most used additives.
Thanks to the low weights and the high mechanical characteristics, the articles in composite materials with carbon fibers made with the heat compression molding technology starting from SMC semi-finished products have been used for some years in the automotive industry for the production of structural components such as, for example, supports for lights, windscreen frames and supporting elements of bodywork parts.
Nevertheless, these articles are not generally used for the manufacture of aesthetic elements of a vehicle, such as, for example, bodywork parts and details of the passenger compartment, due to their rather coarse surface finishing. In particular, an article of composite material made starting from SMC semi-finished products has surface defects such as, for example, porosity and undulations that are even visible to the naked eye and that therefore, despite the excellent mechanical characteristics, make it unsuitable for a use also or exclusively aesthetic.
U.S. Pat. No. 4,855,097 describes a method for making an exterior automotive body panel by compression molding in which a charge containing a curable resin covers from 40% to 80% of the surface area of the molding surface. According to this known document coverages higher than 80% do not permit sufficient flow of charge necessary to provide a slight movement of the same.